The growth of hydrogen-based renewable energy sources as viable alternatives to fossil fuel-based technologies is critically dependent on the development of materials that can significantly influence the efficacy of oxygen electrochemistry; both the oxygen reduction reaction (ORR: O2+4H++4e−2H2O) in fuel cells and the oxygen evolution reaction (OER: 2H2OO2+4H++4e−) in electrolyzers. The development of such materials is guided by two equally important fundamental principles: (i) the catalytic activity for the desired reaction and (ii) long-term stability in hostile electrochemical environments. The methods used to enhance the catalytic activity of the materials for the ORR are diverse, ranging from the alloying and de-alloying of metal catalysts to the synthesis of core-shell catalysts.
The selection of materials for the OER (the anodic half-cell reaction in electrolyzers) is significantly limited, since the metal oxides must have high electronic conductivity, activity, and stability—all very demanding requirements that have severely hampered the utilization of oxide materials in this role. Traditionally, activity of the OER has, for the most part, been correlated primarily in terms of energetic factors whereby the binding energy between the “active sites” and the oxygenated species is assumed to control the kinetics of the OER. Such considerations have formed the backbone of the well-known volcano plot that is generally used to express the kinetics of the OER as a function of more fundamental properties of the oxide materials (e.g. oxygen binding energy, enthalpy of oxide formation, etc. . . . ) which are known as descriptors. It is generally accepted that it is possible to identify materials with unique electronic properties that bind one intermediate not too weakly and another intermediate not too strongly. So far, such energetics-based mechanisms have been used to explain why the most active anode material for the OER in polymer electrolyte membrane electrolyzer (PEM) environments at low pH is the highly conductive RuO2.
Although previous studies have offered important insights into possible relationships between activity of the OER and the oxygen binding energy, no attempts have been made toward an even more important aspect of the electrocatalysts: the fundamental link between activity and stability under an electrolyzer's operating conditions. Without this knowledge, it is very difficult (if not impossible) to build the guiding principles required for the development of new synthesis methods that allow for the design of stable and active real-world commercial anode catalysts. The stability of oxide catalysts has previously been “tested” simply by monitoring the OER current at a certain electrode potential; if the current is found not to change with time (usually within a couple of minutes) the conclusion reached is that the catalyst (typically high surface area materials) is stable. However, this conclusion does not consider that the amount of tested material was always high enough to sustain the apparent kinetics of the OER. The fact that the lifetime of pre-existing oxide materials in electrolyzers is very limited is a clear indication that degradation of oxide and dissolution of active component take place simultaneously with the OER. This is also the case for a RuIr alloy that is currently considered to be one of the most stable anode materials for the OER in PEM based electrolyzers. Several possible explanations have been proposed for a perceived stabilization of Ru atoms by the presence of nearby Ir atoms, including charge transfer coupled with band mixing of the metals' d-bands, modification of the surface dipoles, and the enhanced oxidation of Ir and subsequent passivation of the catalyst surface. However, the materials used to deduce the fundamental principles for the OER catalysts often lead to ambiguities. Commonly used materials include: high surface area particles and oxide macroscopic films, which are often poorly characterized and exhibit uncontrolled defect densities along with poorly defined atomic-level structure.